1. Field of the Invention
The present invention concerns image projectors and, more particularly, optical devices for projecting images or videos, as far as the images or videos have to be displayed onto a surface.
2. Description of Prior Art
Short-distance projectors appeared first in the field of back projection. The reduction in volume necessary for the projection system makes it possible to appreciably reduce the size of the furniture containing the entire system. Examples of systems of this type are described in the patent applications WO 2006/043666 and WO 2006/058884.
The particularity of these systems relates to the fact that the projector is fixed with respect to an image plane of given size. These systems project images of known fixed size at a distance also fixed from the last optical element having a focal power, in this case a curved mirror, that is generally concave. These documents also show that such systems can be constructed from elements all having the same symmetry axis of revolution and a single curved mirror. These mirrors are relatively expensive and the fact of using only one of them is therefore advantageous. In addition, these mirrors are not integrated in the barrel containing the lenses forming the device, which causes assembly difficulties. This assembly is greatly facilitated by the presence of a symmetry axis.
Because of the projection distance and the size of the fixed image, these systems do not require focusing or a zoom function.
These projectors can be used for designing front projection systems. In a front projection system, the projection is not mechanically connected to the projection surface. It is then advantageous to allow focusing by adjustment of the relative positions of certain lenses in order to adjust the sharpness of the image for a given distance between the projector and the projection surface. Such a front projection is known from the patent application WO 2008/11159.
The size of the projection surface may vary during the use of such a front projection system. It is then advantageous to have available a projector with a zoom function for adapting to different sizes of projection surface.